I replaced all glow plugs maybe 1.5 years ago and they've been great and hot. One day the pre-glow light on the dash did not go on, and there was clearly no pre-heat taking place. The car will choke to life with some cranking and starts quickly if I plug in the block heater overnight. I want to find the relay and check it out or boil it or something, but don't know where it is or what it looks like.

It could be something else too I suppose; any comments on this? Thanks!

1983 300SD
375,000 miles